The visual system adjusts to stabilize object color appearance against changes in illumination. This proposal consists of experiments designed to study the adjustment, which is often called color constancy. Understanding color constancy is necessary for a complete theory of human color vision. In addition, color constancy is an example of a larger class of perceptual constancies (e.g. size constancy and shape constancy) that together allow us to perceive a stable physical world. As such, a detailed characterization of the color system may provide insights that generalize to these other perceptual systems. The first set of experiments is designed to characterize performance for natural viewing conditions. In these experiments the stimuli will consist of real illuminated objects. Asymmetric color matching procedures will be used to provide quantitative data. The results of the first set of experiments will be compared with similar data obtained using computer graphics simulations as stimuli. The purpose of this comparison is to establish whether computer graphics technology provides a valid method for studying real world color performance. Because many image parameters are technically difficult to manipulate and control in real images, a valid graphics simulation would make a richer set of experimental manipulations possible. The third set of experiments is designed to measure the action of early color mechanisms for spatially complex stimuli. To isolate early mechanisms, the stimuli will consist of multiple uniformly colored regions. These stimuli have the advantage that they can be described by a relatively small number of parameters. The proposed experiments are designed to provide quantitative tests of qualitative principles that might govern mechanisms of chromatic adaptation and simultaneous color contrast. If valid, these principles allow a finite set of measurements to predict the joint action of the mechanisms for a broad class of images. A final experiment is designed to ask whether early visual mechanisms are independent from higher-level processes that parse the image into objects and illuminants. If so, then the results of the proposed research can be used to determine the degree to which measured real image color constancy can be explained by the action of early mechanisms.